The present invention relates to a novel discotic liquid crystal having a polyfluorinated side chain and a broad and stable liquid crystal phase and to an organic electroluminescence device using the discotic liquid crystal.
Discotic liquid crystal phase is a liquid crystal phase discovered in 1977 by Chandrasekhar, et al. (Pramana, 9, 471 (1977)). As described in their paper entitled xe2x80x9cDiscotic Liquid Crystalsxe2x80x9d (Rep. Prog. Phys., 53, 57 (1990)) and in a paper entitled xe2x80x9cDesign and Synthesis of Discotic Liquid Crystal Moleculesxe2x80x9d by Shunsuke Takenaka (Japanese Chemical Society, Seasonal Publication, General Review, vol. 22, pp. 60+), the discotic liquid crystal phase is found in compounds having a disk-shaped core and a plurality of relatively long chains connected to the core. Such compounds may be classified into various types according to their core structure, inclusive of derivatives of hexa-substituted benzene and tri-substituted benzene; derivatives of phthalocyanine and porphyrin; derivatives of triphenylene, truxene and pyrylium, respectively; tribenzocyclononene derivatives, azacrown derivatives, and cyclohexane derivatives.
Based on the structural characteristic of a discotic liquid crystal, several reports have been made suggesting application thereof to devices. A systems including conjugated xcfx80-electrons, as found in derivatives of phthalocyanine or triphenylene, can provide a channel for electrons (or holes) (Piechocki, et al., J. Am. Chem. Soc., 104, pp. 5245 (1982)). Further, a system including an annular core, as found in an aza-crown derivative, can provide a molecular channel using the central spacing thereof as a selective molecular passage (Lehn, et al., J. Chem. Soc., Chem. Commun., pp. 1794 (1985)).
On the other hand, since 1987 when T. W. Tang et al. proved that a high luminance light emission was achieved by a low voltage drive of a laminate of their films of a fluorescent metal chelate complex and a diamine molecule, extensive research has been made on organic electroluminescence devices (hereinafter, the term xe2x80x9celectroluminescencexe2x80x9d is sometimes abbreviated as xe2x80x9cELxe2x80x9d according to a common usage in the field) as luminescence or light emission devices having a high speed responsiveness and a high efficiency. An organic EL device is a carrier injection-type self-light emission device utilizing luminescence caused at the time of recombination of electrons and holes having reached the luminescence layer.
FIGS. 2 and 3 respectively illustrate a laminate structure of an ordinary organic EL device. Referring to FIG. 2 (or FIG. 3), an EL device includes a cathode metal electrode 21 (or 31) and an anode transparent electrode 24 (or 35) disposed on a transparent substrate 25 (or 36) for taking out luminescent light. Organic compound layers, each having a thickness on the order of several hundred xc3x85 (angstoms), are sandwiched between the electrodes. The cathode may generally comprise a metal having a small work function, such as aluminum, aluminum-lithium alloy, magnesium-silver alloy, etc. The anode may comprise a conductive material having a large work function, such as indium tin oxide (ITO). The organic compound layers, may ordinarily have a two layer structure including a luminescence layer 22 and a hole-transporting layer 23 as shown in FIG. 2 or a three layer structure including an electron-transporting layer 32, a luminescence layer 33 and a hole-transporting layer 34 as shown in FIG. 3.
The hole-transporting layer has a function of effectively injecting holes from the anode into the luminescence layer, and the electron-transporting layer has a function of effectively injecting electrons from the cathode into the luminescence layer. The hole-transporting layer also has a function of confining electrons, and the electron-transporting layer also has a function of confining holes, respectively, into the luminescence layer, i.e., carrier-blocking functions for enhancing the luminescence efficiency. For these carrier-transporting layers, inclusive of the hole-transporting layer and the electron-transporting layer, a charge-transporting performance, particularly a carrier mobility, may be regarded as an important property. An organic compound in an amorphous state may generally exhibit a carrier mobility on the order of 10xe2x88x925 cm2/V.sec, which cannot be said to represent a sufficient transporting performance. It is believed that if the mobility of a carrier-transporting layer is increased, a larger amount of carrier can be injected into the luminescence layer to enhance the luminescence efficiency, and simultaneously the thickness of the carrier-transporting layer (generally having a thickness on the order of several hundred xc3x85) can be increased (to a thickness up to ca. 1 xcexcm), so that it becomes possible to effectively prevent a short circuit between the electrodes sandwiching the organic layers and provide an improved productivity.
At present, in order to achieve a higher efficiency organic EL device, extensive work toward the development of various compound materials for the carrier-transporting layers has been made. Along with the activity, some proposal has been made to achieve a higher mobility by imparting mesomorphism to organic compounds forming carrier-transporting layers. Organic films generally used in organic EL devices are in an amorphous state and have no regularity regarding molecular alignment. In contrast thereto, some organic compounds in a liquid crystal state, i.e., having some order of molecular alignment, have been found to show a high mobility, thus calling attention.
For example, Haarer, et al. observed that a long chain triphenylene compound, a representative discotic liquid crystal material, exhibited a high hole mobility of 10xe2x88x921 cm2/V.sec (Nature, vol. 371, p. 141 (1994)). Further, Haarer, et al. examined a relationship between hole mobility and molecular alignment order in columnar phase for a series of triphenylene-type discotic liquid crystals and reported that a higher order provided a higher mobility (Adv. Mater., vol. 8, p. 815 (1996)). Thus, a molecular alignment control advantageous for carrier transportation is expected to be achieved by utilizing spontaneous alignment of mesomorphic organic compounds, thus providing excellent carrier-transporting materials. On the other hand, organic EL devices involve problems regarding durability, such as deterioration of luminescence performance due to moisture and due to reaction between organic compound layers.
Some discotic liquid crystal compounds having polyfluorinated side chains have been reported in Liquid Crystal, vol. 19, No. 6, pp. 759-764 (1995). More specifically, three species of triphenylene derivatives (5a, 5b and 5c), each having 6 polyfluorinated side chains, are shown in FIG. 2 at page 760 of the above report. These compounds have an intermediate carboxyl group in their side chains and do not cause transformation from the discotic columnar phase to clarifying point (Iso), but reach a decomposition point on temperature increase as shown in a table at an upper left portion of page 761, so that they cannot be regarded as stable discotic liquid crystal compounds.
A generic object of the present invention is to solve the above-mentioned problems of the prior art.
A more specific object of the present invention is to provide a novel discotic liquid crystal compound having a stable and broad discotic liquid crystal phase.
Another object of the present invention is to provide an organic electroluminescence device exhibiting stable and good luminescence performance by using the liquid crystal compound.
According to the present invention, there is provided a discotic liquid crystal compound represented by formula (1) below:
Ar"Parenopenst"Xxe2x80x94R)nxe2x80x83xe2x80x83(1),
wherein Ar denotes a group of 2,3,5,6-benzoquinone-tetra-yl, 2,3,4,6,7,8-anthraquinone-hexa-yl, 2,3,6,7,10,1 1-triphenylene-hexa-yl, 2,3,7,8,12,13-truxene-hexa-yl, 2,3,6,7,10,11-tricycloquinazoline-hexa-yl, or 1,2,5,6,8,9,12,1 3-dibenzopyrene-octa-yl; X denotes a single bond, an oxygen atom, a sulfur atom, xe2x80x94OOCxe2x80x94 or xe2x80x94COOxe2x80x94; R denotes a linear or branched alkyl group having 3-20 carbon atoms, of which at least 2 hydrogen atoms have been replaced with fluorine atoms and of which one methylene group can be replaced with an oxygen atom, a sulfur atom, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94; and n is an integer of 4, 6 or 8 corresponding to a valence of the group Ar. The discotic liquid crystal of the present invention is essentially different from the above-mentioned discotic liquid crystal compounds disclosed in Liquid Crystal having polyfluorinated side chains that include intermediate carboxyl (i.e., ester) groups.
The present invention further provides an organic electroluminescence device including a layer comprising the discotic liquid crystal of the present invention.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.